I haven't seen this issue officially posted yet, though it has been mentioned a few times on the Sims forums.
I've started a new game to see if this bug keeps occuring. I sent a sim every day at the same time, for 7 days in a row, to the community lots, the 2 big parks and the coast areas (with the museum, gym, lounge/bar, library) in both worlds. The rocks spawned on the first day, my sim could dig them just fine, but after that they didn't spawn again. The areas around residences aren't affected by this, it's just the community lots. I'm not sure if everyone shares this bug, I wouldn't have noticed it if I hadn't been playing a sim that loved collecting things.
I've been taking screenshots while sending my sim to the community lots, those can be found in my online album: http://1drv.ms/1vDxftj.
It would be great if anyone else could also confirm the bug, it's not a game breaking one, but if you have the preference to send your sims to community lots for collecting, then it does impact gameplay.
@NevisDreams Merged. The problem is that they do spawn but only if you play the lot for quite a while.
You're right, the rocks finally respawned for me, it took around 18 sim hours.
